Electron beam radiotherapy calibrator. These graphite plates are used in a calorimeter that measures the strength of electron beams produced by electron beam radiotherapy equipment. The calorimeter determines how much of these electrons will be absorbed by the patient when undergoing this radiotherapy, so that the equipment can be fine-tuned (calibrated) if the dose of electrons absorbed is found to be slightly too high or too low for the most effective treatment. Photographed at the National Physical Laboratory in Teddington, UK.